1. Technical Field
This invention relates to an accelerometer, and in particular to a microelectromechanical (MEMS) accelerometer of relatively small dimensions and low cost.
2. Background Information
MEMS accelerometers are in widespread use, for example in automotive and other applications. One area in which they are used is in global positioning applications, to provide back-up information indicative of the movement of a vehicle for use during short intervals in which satellite communications links are temporarily interrupted. However, it will be appreciated that this is merely one possible application in which a MEMS accelerometer may be used, and that the invention is not restricted in this regard.
U.S. Pat. No. 7,047,808 describes a MEMS accelerometer suitable for use in such applications. The accelerometer comprises a proof mass of plate-like form surrounded by a ring-like support member. The support member and the proof mass are substantially coplanar, and the proof mass is connected to the support member by means of a series of mounting legs that are formed integrally with the mass and the support member. Each leg extends in a direction substantially perpendicular to a direction in which the accelerometer is sensitive to accelerations. In use, the accelerometer is mounted upon an object, the motion of which is to be monitored, the accelerometer being mounted in such a manner that the support member is rigidly secured to the object, the accelerometer being correctly orientated such that the mounting legs each extend in a direction substantially perpendicular to the sensing direction. If the object is accelerated in the sensing direction, it will be appreciated that the inertia of the proof mass will result in the proof mass moving relative to the support member, the mounting legs flexing and applying a restoring force urging the proof mass back towards its rest position.
In order to permit relative movement between the proof mass and the support member to be sensed and thereby to permit an electrical output indicative of the acceleration to be produced, the proof mass is provided with several groups of capacitor fingers, each of which extends substantially parallel to the mounting legs. Similarly, the support member is provided with several groups of capacitor fingers, the fingers of each group being interdigitated with the capacitor fingers of a corresponding group associated with the proof mass. Movement of the proof mass relative to the support member results in relative movement of adjacent ones of the interdigitated fingers. By taking appropriate capacitance measurements, the position or movement of the proof mass relative to the support member can be determined. As the movement of the proof mass relative to the support member arises in use from the object to which the accelerometer is mounted experiencing an acceleration, it will be appreciated that the movement output is also indicative of the experienced acceleration.
Other, similar devices are described in, for example, U.S. Pat. No. 7,562,573 and U.S. Pat. No. 7,267,006.
In an arrangement of this type there is a need to make drive electrical connections to the various groups of capacitor fingers provided on the support member. Thus, where the accelerometer includes, for example, four groups of capacitor fingers associated with the support member, four such connections need to be made in addition to the required ground and output connections. The provision of such connections takes up space, resulting in the accelerometer being relatively large, and there are also cost implications associated with the need to provide such connections, and additional manufacturing complexities.
It is an object of the invention to provide an accelerometer in which at least some of the disadvantages with known arrangements are overcome or are of reduced effect.
There is a desire to provide an accelerometer device that is sensitive to accelerations in two or more perpendicular directions, and it is another object of the invention to provide an accelerometer suitable for use in such a device.